FIG. 1 shows a prior art structure of a circuit in a pixel (pixel circuit) in an active matrix organic EL display device.
When a gate line (Gate) extending along the horizontal direction is set at a high level, a selection TFT (thin film transistor) 2 is switched ON. While in this state, a data signal voltage corresponding to the display brightness is applied on a data line (Data) extending along the vertical direction so that the data signal voltage is stored in a storage capacitor C. Because the data signal voltage stored in the storage capacitor is applied to a gate of a driver TFT 1, the driver TFT 1 supplies a drive current corresponding to the data signal voltage to an organic EL element, and light is emitted from the organic EL element. In FIG. 1, PVdd represents a high-voltage side power supply of the panel and CV represents a low-voltage side power supply of the panel.
FIG. 2 shows a prior art structure of a display panel and input signals. As shown in FIG. 2, pixel portions are placed on the display panel in a matrix form. First, a gate driver sets the gate line at the high level and a source driver supplies, on the data line, a data signal voltage for pixels on a horizontal line which is being selected. Then, the gate line is returned to the low level. With this process, the data signal voltage is stored in each pixel portion, and display according to the data signal voltage for the pixel is realized in each pixel until the next frame.
An image data signal, a horizontal synchronization signal, a dot clock, and other drive signals are supplied to the source driver, and the horizontal synchronization signal, a vertical synchronization signal, and other drive signals are supplied to the gate driver.
The amount of light emission from the organic EL element and the drive current are approximately in a proportional relationship. Normally, a voltage (Vth) is applied between the gate of the driver TFT 1 and PVdd so that a drain current starts to flow around a black level of an image. As an amplitude of the data signal voltage, an amplitude is set which causes a predetermined brightness to be achieved near a white level.
FIG. 3 is a prior art diagram showing a relationship between an input signal voltage of the driver TFT 1 (voltage on the data line Data) and current flowing through the organic EL element (CV current). As described above, the CV current corresponds to the emission brightness of the organic EL element. By determining the data signal voltage so that Vb is set as the black level voltage and Vw is set as the white level voltage, a suitable gray scale control can be achieved in the organic EL element.
Because a current when a pixel is driven with a certain data signal voltage depends on characteristics of the driver TFT 1, such as Vth and a slope (μ) of the V-I curve, the brightness would be uneven when the characteristics of the driver TFTs 1 in the panel such as Vth and μ vary among pixels. In order to correct the brightness unevenness, it is necessary to input, to each pixel, a data signal voltage such that the same brightness is achieved corresponding to the same image data signal. For this purpose, one or a number of predetermined pixels of a panel is caused to emit light at several signal levels, and the V-I curve of the TFT has been determined based on the panel current at each signal level (refer to, for example, U.S. Patent Application Publication No. 2004/0150592 and WO 2005/101360).
Currently, the current value for each pixel, although dependent on the efficiency of the organic EL element to be used and the pixel density, is few microamperes (μA) or less even when the organic EL element is caused to emit bright light at a typically used level, and in particular, a current of 1 μA or less must be measured in order to determine the variation in the current value near the black level. In this case, noise coming from the outside of the panel and noise flowing from the driver circuits in the panel to the CV and PVdd power supplies become factors that reduce the measurement precision.